In recent years, a demand concerning flatness of a wafer such as a silicon wafer has been increased more than ever, and fabricating a wafer having higher flatness in single-side polishing has been demanded. Further, to provide a wafer having high flatness with excellent reproducibility, a polishing head including a rubber film which holds the wafer, a space section which is in contact with the rubber film, and an incompressible fluid which is sealed in the space section is used (see, e.g., Patent Literature 1).
According to such a polishing head, since a shape of a surface of the rubber film can be appropriately adjusted by the incompressible fluid, when the surface of the rubber film is appressed against an entire back surface of the wafer to press the wafer, polishing can be carried out. Consequently, a stock removal of the wafer can be uniformed on an entire polishing surface, and the wafer with high flatness can be fabricated. Further, since the shape of the surface of the rubber film which adsorbs the wafer can be controlled to be constant by the incompressible fluid, the wafer with high flatness can be provided with excellent reproducibility.
However, to uniform the shape of the surface of the rubber film which adsorbs the wafer, the incompressible fluid must be sealed in the space section in the polishing; head without mixing air in manufacture of the polishing head. That is because a pressure in a portion where the air is present is different from those in other portions when the air is mixed, the shape of the surface of the rubber film cannot be controlled to be constant, and the wafer cannot be uniformly pressed. Furthermore, when the air is mixed, a volume of the incompressible fluid which is to be sealed in the polishing head largely varies, and the shape of the polished wafer also largely varies. Thus, in manufacture of the polishing head, the air must be prevented from remaining particularly in the space section where the incompressible fluid is sealed in.
Thus, to prevent the air from being mixed, parts of the polishing head are immersed in the incompressible fluid, and the polishing head is manually assembled in the incompressible fluid in some cases. However, according to this technique, controlling an amount of the incompressible fluid to be sealed in is difficult. Moreover, since the polishing head is assembled in the incompressible fluid, workability is considerably degraded. Additionally, a polishing head which is used for polishing of a large-diameter wafer having a diameter of 300 mm or more has a large size and a very heavy weight, and hence a problem arises in a safety aspect as well as the workability. Further, when the incompressible fluid to be sealed in is harmful to human bodies, work itself is impossible.
On the other hand, as described, below, there is also a technique to assemble the polishing head in air rather than the incompressible fluid. According to this technique, first, as shown in an upper part of FIG. 6, a rigid ring 102, an intermediate plate 104, and a rubber film 103 are assembled, and a space section 105 where an incompressible fluid is sealed in is formed in the polishing head. Furthermore, in the intermediate plate 104, an inlet 107 through which the incompressible fluid is poured and an outlet 108 through which air is discharged are formed. Then, the inside of the space section 105 is depressurized. Thereafter, the incompressible fluid is poured into the space section 105 from the inlet 107 communicating with the space section 105 and, at the same time, the air remaining in the space section 105 is discharged from the outlet 108. As shown in a lower part of FIG. 6, after a sufficient amount of the incompressible fluid 106 is poured, the inlet 107 and the outlet 108 are closed with lids 109 (this technique will be also referred to as a depressurizing and sealing method hereinafter). According to this depressurizing and sealing method, assembling the polishing head in the air enables greatly improving the workability. Moreover, an amount of the incompressible fluid to be sealed in can be easily controlled.